This applies to the Rednet 1 and Rednet 2
The analogue line-level inputs and outputs on the Rednet 1 & 2 are made via back panel mounted 25-pin D connectors. The pin connections used follow the AES59 "DigitalCombined I/O" convention which can be found here.
Rednet 1 Rear Panel:
Rednet 2 Rear Panel: